Editing Surveying (section)

==Techniques==
[[File:Brunton.JPG|thumb|alt= A compass with extra sights for measuring bearings. |A standard Brunton Geo [[compass]], still used commonly today by geographers, geologists, and surveyors for field-based measurements]]

Surveyors determine the position of objects by measuring angles and distances. The factors that can affect the accuracy of their observations are also measured. They then use this data to create vectors, bearings, coordinates, elevations, areas, volumes, plans and maps. Measurements are often split into horizontal and vertical components to simplify calculation.
GPS and astronomic measurements also need measurement of a time component.

===Distance measurement===
[[File:Field-Map birdie.jpg|thumb|left|upright|alt= A Woman with a backpack holding a laser rangefinder, a handheld GPS and a Tablet computer. |Example of modern equipment for surveying ([[Field-Map]] technology): [[GPS]], [[laser rangefinder]] and field computer allows surveying as well as [[cartography]] (creation of map in real-time) and field data collection.]]
Before [[Rangefinder|EDM]] (Electronic Distance Measurement) laser devices, [[distance]]s were measured using a variety of means. In pre-colonial America Natives would use the "bow shot" as a distance reference ("as far as an arrow can slung out of a bow", or "flights of a Cherokee long bow").<ref>{{Cite web | url=https://apps.mht.maryland.gov/Medusa/PDF/NR_PDFs/NR-311.pdf | title=National Register of Historic Places - Registration form for Antietam Village Historic District}}</ref> Europeans used chains with links of a known length such as a [[Gunter's chain]], or measuring tapes made of steel or [[invar]]. To measure horizontal distances, these chains or tapes were pulled taut to reduce sagging and slack. The distance had to be adjusted for heat expansion. Attempts to hold the measuring instrument level would also be made. When measuring up a slope, the surveyor might have to "break" (break chain) the measurement- use an increment less than the total length of the chain. [[Surveyor's wheel|Perambulators]], or measuring wheels, were used to measure longer distances but not to a high level of accuracy. [[Tacheometry]] is the science of measuring distances by measuring the angle between two ends of an object with a known size. It was sometimes used before to the invention of EDM where rough ground made chain measurement impractical.

===Angle measurement===
Historically, horizontal angles were measured by using a [[Circumferentor|compass]] to provide a magnetic bearing or azimuth. Later, more precise scribed discs improved angular resolution. Mounting telescopes with [[reticle]]s atop the disc allowed more precise sighting (see [[theodolite]]). Levels and calibrated circles allowed the measurement of vertical angles. [[Vernier scale|Vernier]]s allowed measurement to a fraction of a degree, such as with a turn-of-the-century [[Transit (surveying)|transit]].

The [[plane table]] provided a graphical method of recording and measuring angles, which reduced the amount of mathematics required. In 1829 [[Francis Ronalds]] invented a [[Reflecting instrument#Surveying sector|reflecting instrument]] for recording angles graphically by modifying the [[Octant (instrument)|octant]].<ref>{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location=London}}</ref>

By observing the bearing from every vertex in a figure, a surveyor can measure around the figure. The final observation will be between the two points first observed, except with a 180° difference. This is called a ''close''. If the first and last bearings are different, this shows the error in the survey, called the ''angular misclose''. The surveyor can use this information to prove that the work meets the expected standards.

===Leveling===
{{main|Levelling}}
[[File:Tide station leveling.jpg|thumbnail|alt= A woman setting up an optical level on a tripod. |A Center for Operational Oceanographic Products and Services staff member conducts tide station leveling in support of the US Army Corps of Engineers in Richmond, Maine.]]

The simplest method for measuring height is with an [[altimeter]]&nbsp;using air pressure to find the height. When more precise measurements are needed, means like precise levels (also known as differential leveling) are used. When precise leveling, a series of measurements between two points are taken using an instrument and a measuring rod. Differences in height between the measurements are added and subtracted in a series to get the net difference in elevation between the two endpoints. With the [[Global Positioning System]] (GPS), elevation can be measured with satellite receivers. Usually, GPS is somewhat less accurate than traditional precise leveling, but may be similar over long distances.

When using an optical level, the endpoint may be out of the effective range of the instrument. There may be obstructions or large changes of elevation between the endpoints. In these situations, extra setups are needed. ''Turning'' is a term used when referring to moving the level to take an elevation shot from a different location. To "turn" the level, one must first take a reading and record the elevation of the point the rod is located on. While the rod is being kept in exactly the same location, the level is moved to a new location where the rod is still visible. A reading is taken from the new location of the level and the height difference is used to find the new elevation of the level gun, which is why this method is referred to as ''differential levelling''. This is repeated until the series of measurements is completed. The level must be horizontal to get a valid measurement. Because of this, if the horizontal crosshair of the instrument is lower than the base of the rod, the surveyor will not be able to sight the rod and get a reading. The rod can usually be raised up to {{convert|25|ft}} high, allowing the level to be set much higher than the base of the rod.

===Determining position===
The primary way of determining one's position on the Earth's surface when no known positions are nearby is by astronomic observations. Observations to the Sun, Moon and stars could all be made using navigational techniques. Once the instrument's position and bearing to a star is determined, the bearing can be transferred to a reference point on Earth. The point can then be used as a base for further observations. Survey-accurate astronomic positions were difficult to observe and calculate and so tended to be a base off which many other measurements were made. Since the advent of the GPS system, astronomic observations are rare as GPS allows adequate positions to be determined over most of the surface of the Earth.

===Reference networks===
{{main|Geodetic network}}
[[File:Traversing and Offsetting DP 79152.png|thumb|alt= A diagram of survey markers running along a shoreline. |A survey using traverse and offset measurements to record the location of the shoreline shown in blue. Black dashed lines are traverse measurements between reference points (black circles). The red lines are offsets measured at right angles to the traverse lines.]]

Few survey positions are derived from the first principles. Instead, most surveys points are measured relative to previously measured points. This forms a reference or ''control'' network where each point can be used by a surveyor to determine their own position when beginning a new survey.

Survey points are usually marked on the earth's surface by objects ranging from small nails driven into the ground to [[Trig beacon|large beacons]] that can be seen from long distances. The surveyors can set up their instruments in this position and measure to nearby objects. Sometimes a tall, distinctive feature such as a steeple or radio aerial has its position calculated as a reference point that angles can be measured against.

''[[Triangulation]]'' is a method of horizontal location favoured in the days before EDM and GPS measurement. It can determine distances, elevations and directions between distant objects. Since the early days of surveying, this was the primary method of determining accurate positions of objects for [[topographic]] maps of large areas. A surveyor first needs to know the horizontal distance between two of the objects, known as the [[Baseline (surveying)|baseline]]. Then the heights, distances and angular position of other objects can be derived, as long as they are visible from one of the original objects. High-accuracy transits or theodolites were used, and angle measurements were repeated for increased accuracy. See also [[Triangulation in three dimensions]].

''Offsetting'' is an alternate method of determining the position of objects, and was often used to measure imprecise features such as riverbanks. The surveyor would mark and measure two known positions on the ground roughly parallel to the feature, and mark out a baseline between them. At regular intervals, a distance was measured at right angles from the first line to the feature. The measurements could then be plotted on a plan or map, and the points at the ends of the offset lines could be joined to show the feature.

[[Traverse (surveying)|''Traversing'']] is a common method of surveying smaller areas. The surveyors start from an old reference mark or known position and place a network of reference marks covering the survey area. They then measure bearings and distances between the reference marks, and to the target features. Most traverses form a loop pattern or link between two prior reference marks so the surveyor can check their measurements.

====Datum and coordinate systems====
{{main|Geodetic datum}}

Many surveys do not calculate positions on the surface of the Earth, but instead, measure the relative positions of objects. However, often the surveyed items need to be compared to outside data, such as boundary lines or previous survey's objects. The oldest way of describing a position is via latitude and longitude, and often a height above sea level. As the surveying profession grew it created Cartesian coordinate systems to simplify the mathematics for surveys over small parts of the Earth. The simplest coordinate systems assume that the Earth is flat and measure from an arbitrary point, known as a 'datum' (singular form of data). The coordinate system allows easy calculation of the distances and direction between objects over small areas. Large areas distort due to the Earth's curvature. North is often defined as true north at the datum.

For larger regions, it is necessary to model the shape of the Earth using an ellipsoid or a geoid. Many countries have created coordinate-grids customized to lessen error in their area of the Earth.

===Errors and accuracy===

A basic tenet of surveying is that no measurement is perfect, and that there will always be a small amount of error.<ref>{{cite book|last1=Kahmen|first1=Heribert|last2=Faig|first2=Wolfgang|title=Surveying|date=1988|publisher=de Gruyter|location=Berlin|isbn=3-11-008303-5|page=9|url=https://books.google.com/books?id=5AFNBqbiLzoC&q=surveying|access-date=10 August 2014}}</ref> There are three classes of survey errors:
*''Gross errors or blunders:'' Errors made by the surveyor during the survey. Upsetting the instrument, misaiming a target, or writing down a wrong measurement are all gross errors. A large gross error may reduce the accuracy to an unacceptable level. Therefore, surveyors use redundant measurements and independent checks to detect these errors early in the survey.
*''Systematic:'' Errors that follow a consistent pattern. Examples include effects of temperature on a chain or EDM measurement, or a poorly adjusted spirit-level causing a tilted instrument or target pole. Systematic errors that have known effects can be compensated or corrected.
*''Random:'' Random errors are small unavoidable fluctuations. They are caused by imperfections in measuring equipment, eyesight, and conditions. They can be minimized by redundancy of measurement and avoiding unstable conditions. Random errors tend to cancel each other out, but checks must be made to ensure they are not propagating from one measurement to the next.

Surveyors avoid these errors by calibrating their equipment, using consistent methods, and by good design of their reference network. Repeated measurements can be averaged and any outlier measurements discarded. Independent checks like measuring a point from two or more locations or using two different methods are used, and errors can be detected by comparing the results of two or more measurements, thus utilizing [[Redundancy (engineering)|redundancy]].

Once the surveyor has calculated the level of the errors in his or her work, it is [[adjustment of observations|adjusted]]. This is the process of distributing the error between all measurements. Each observation is weighted according to how much of the total error it is likely to have caused and part of that error is allocated to it in a proportional way. The most common methods of adjustment are the [[Nathaniel Bowditch|Bowditch]] method, also known as the compass rule, and the [[least squares|principle of least squares]] method.

The surveyor must be able to distinguish between [[accuracy and precision]]. In the United States, surveyors and civil engineers use units of feet wherein a survey foot breaks down into 10ths and 100ths. Many deed descriptions containing distances are often expressed using these units (125.25&nbsp;ft). On the subject of accuracy, surveyors are often held to a standard of one one-hundredth of a foot; about 1/8&nbsp;inch. Calculation and mapping tolerances are much smaller wherein achieving near-perfect closures are desired. Though tolerances will vary from project to project, in the field and day to day usage beyond a 100th of a foot is often impractical.